ARTIFICIAL CLOUD-MODIFICATION, AND PRECIPITATION 
of the completion of a cloud-seeding mission [4]. Since 
this snow fell in exactly the area where the seeding had 
been done and was somewhat local in character, it 
seemed that it had resulted from the artificial nuclea- 
tion processes. However, a detailed analysis of the in- 
formation secured during the mission, especially the 
radar pictures, indicated that this was definitely not the 
case. Actually, the measured winds carried the seeded 
clouds outside the precipitating area within a couple 
of hours. A layer of altostratus clouds, based at 4000 
ft and extending to 8700 ft, where the temperature was 
—18C, was seeded with dry-ice pellets at the rate of 
four pounds per mile. The first task in analyzing the 
data was to determine the exact place of the seeding. 
Since the seeding run was made from 1443:45 to 1445:15 
EST, its exact location could be found from the pictures 
of the radarscope during that period. The seeding air- 
eraft’s location was easily marked by its coded beacon 
signal during the run. Before the seeding, it had been 
determined that the cloud deck was composed wholly of 
supercooled water droplets, but ice crystals were ob- 
served to form immediately along the seeded line as the 
dry ice was dropped. Observers in an aircraft flying 
12,000 ft above the cloud top reported that the seeding 
aircraft looked like a snowplow moving across the cloud 
deck. A trough about 300 ft deep resulted as the con- 
version to ice crystals took place while, along the perim- 
eter of the affected area, clouds built up an estimated 
100 ft. This trough, which widened to 2144 miles, was 
easily recognizable for about 45 min, after which mixing 
dissipated the lines of demarcation between the treated 
clouds and the rest of the deck. The first radar echo 
resulting from the seeding was observed about 30 min 
after the drop (see Fig. 2). The position of this echo 
Fie. 2.—Radarscope photograph, January 21, 1948 
(first seeding). 
agreed exactly with the location of the seeded area as 
indicated by the observing aircraft and also with its 
position computed from the winds aloft. This seeding 
echo was approximately three miles wide by about eight 
miles long and never grew any larger. By 1540, about 
one hour after the seeding, it had almost completely 
disappeared. It can be concluded, then, that the seeding 
resulted in the production of a precipitation area from 
which a limited radar echo was observed for about 30 
min. This echo, however, was not unique at the time of 
seeding, since other echoes of similar size and natural 
origin were also indicated. This seeded area was ob- 
237 
served to move with the prevailing winds at the cloud 
altitude and, by the time it disappeared, it was some 
twenty miles to the east of the rain-gage-network area. 
At 1540, a new echo directly over the rain-gage area 
was observed to form, completely independent of the 
seeded echo. Snow continued to fall for some time from 
the unseeded cloud deck as it passed over the network, 
but it was not until approximately 1800 that enough had 
fallen to affect the rain gages. A complete series of 
photographs like Fig. 2 is given for this test in Weather 
Bureau Research Paper No. 30 [4]. 
In summary, to quote from this paper: 
A definite change in texture of the supercooled cloud deck 
was caused by the first seeding. Shortly thereafter, a radar 
echo was noted to be associated with the seeded area and 
snow was observed underneath it. However, at the same time 
in the immediate vicinity, snow echoes were existing or were 
forming. Within one hour after the seeding, the area could no 
longer be distinguished by the observers flying in the aircraft 
above the cloud. The snow echo as indicated on the radar 
scope had disappeared also. At no time was there a definite 
hole broken through the cloud as a result of the seeding, al- 
though there were large breaks in the clouds nearby. 
A study of the pictures presented in that report in- 
dicates that some few ice crystals remained in the 
seeded area and acted as an obstruction to visibility 
even when the snow was reported to be falling from 
beneath the seeded line. Whether or not the seeding 
resulted in a hole in the cloud deck does not seem to be 
a meaningful question in this case, since the same pic- 
tures indicate very large natural breaks immediately 
bordering the seeded area. 
Another seeding conducted in the Wilmington area, 
that of October 5, 1948, was also successful in produc- 
ing a radar echo in the exact pattern of the seeding. 
On this particular day, the cloud deck extended from 
8000 ft to 11,600 ft, where the temperature was —5C. 
Dry-ice pellets were dropped at the rate of five pounds 
per mile in an L pattern. Immediately after the seeding 
run, a texture change and the horizontal diffusion of ice 
crystals were observed. The area continued to spread 
slowly and a shght boiling effect was observed along 
the extremities of the pattern. At 1445, almost exactly 
30 min after the dry ice was dropped, a small radar 
echo was observed from the seeded area. Within 5 
min this echo assumed the shape of the L pattern 
(Fig. 3) but within 5 min thereafter completely disap- 
peared. Observers flying underneath the seeded pat- 
tern reported that light rain fell at about the time of 
the maximum radar echo. They indicated that the 
rain was of short duration and possibly did not reach 
the ground. 
On this day, project pilots indicated that a visual 
descent would have been possible in the seeded area at 
about the time of maximum dissipation, which was 
also the time of maximum radar echo. However, this 
dissipation was not considered particularly significant 
in view of the fact that much larger natural openings 
existed within a few miles of the seeded area. (See 
Fig. 4, which shows the seeded L in the upper middle 
